The International Terrestrial Reference System describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way t
ITRF reference stations
Prime meridian (Greenwich)
Videos
The Greenwich meridian is a prime meridian, a geographical reference line that passes through the Royal Observatory, Greenwich, in London, England. From 1884 to 1974, the Greenwich meridian was the in
Laser projected from the Royal Observatory, marking the prime meridian of Greenwich
Tourists queuing to take pictures on the brass line marking the prime meridian at the Royal Observatory, Greenwich
A GPS receiver at the marking strip of the Greenwich Meridian in front of the Royal Observatory. The indicated longitude is not exactly zero because the geodetic zero meridian on a geocentric reference ellipsoid (which is what GPS positioning yields, using the IERS Reference Meridian) is 102 metres east of this strip.